Chemical processing industries employ condensed-ring aromatic hydrocarbon liquids (for example, DOWTHERM) to convey heat, either supplying heat to endothermic chemical processes or carrying heat away from exothermic processes. The usefulness of such heat-carrying media is limited to temperatures below about 370 C, because their rate of decomposition (with production of a carbon residue) becomes too rapid at higher temperatures. For temperatures higher than about 370 C, chemical processing industries are obliged to use far less convenient heat carriers, such as molten salts or mercury vapor. There is need for a chemically stable, easy-to-handle heat carrier, suitable for use across a wide spectrum of temperatures including those beyond reach today even through use of molten salts or mercury vapor. Endothermic chemical transformation processes are under consideration for temperatures beyond 1,000 C (heat from nuclear fission or solar radiation sometimes being proposed to promote these processes); a medium for delivering heat at such temperatures would bring these suggestions closer to realization.
Chemical processing industries may also advantageously use means for collecting and distributing heat widely, across relatively long distances, throughout a large ground area typically covered by a major chemical processing complex, which typically includes many operations, some exothermic and others endothermic.
An ideal carrier for conveying heat long distances and at high temperatures would be a chemically stable powder. Comminuted solids, including both fine powders and larger moieties, have long been in use for conveying heat to endothermic processes, for example, in supplying heat to the cracking step found in two process types for catalytically cracking gas oils. One type, fluid catalytic cracking (FCC), employs a fine catalytic powder (herein called “FCC catalyst”) to carry heat from a combustion step (removing carbon from the catalyst) to an endothermic cracking step. In some embodiments of the FCC cracking process, the two steps are mounted side-by-side, so that at least a portion of the travel of FCC catalyst between the two steps is horizontal. Note, however, that horizontal travel distance is small relative to ground occupied by a petroleum refinery that the FCC cracking process serves. A second type, gravitating-bed cracking, employs a catalyst in form of “beads” of about 3 mm (⅛ inch) diameter. In substantially all modern embodiments of gravitating-bed cracking, the two steps (cracking and catalyst-regenerating combustion) are mounted vertically, one above the other, catalyst beads experiencing substantially no sideways travel.
Wider usefulness of a fine or coarse powder for carrying heat requires better means for conveying a hot powder horizontally, even when distance of travel is relatively large, with something approaching the ease of moving a condensed-ring-hydrocarbon liquid horizontally (or a molten salt or mercury vapor). Conventional arrangements for pneumatic conveying of a dry powder horizontally within a pipe—the powder moving either in the “dilute phase” (whereby powder particles move either singly or in relatively small clumps or packets) or in the “dense phase” (whereby particles move in “plugs” separated by “empty” intervals, occupied by gas)—suffer two handicaps: first, a pressure gradient necessary to sustain the powder/gas motion can be large; second, if a hot powder be employed as a carrier of heat, either conveying gas must be heated to substantially the temperature of the powder or a reduction in this temperature must be accepted through introduction of cooler gas.
These disabilities also characterize a system for pneumatic powder conveying disclosed in U.S. Pat. No. 3,268,264. U.S. Pat. No. 3,268,264 teaches how to cause a dry granular material to flow through a horizontal duct fitted with a porous floor, beneath which is situated a plenum supplied with a gas that fluidizes the material. Means are provided for increasing pressure of the material, to make good a drop in this pressure between a granular-material-entry end and granular-material-exit end of the duct. For purpose of engineering design, this pressure drop can be estimated by integrating point pressure gradients along the duct; these can be estimated by applying a standard correlation for flow of air and water through a horizontal duct of similar dimensions. Recall that air and water, when flowing together in an enclosed, horizontal duct, separate into two “layers”: water flows in a lower layer, and air flows in an upper layer occupying a “supernatant” space above the flowing water. The air speed is significantly greater that the speed of the water. U.S. Pat. No. 3,268,264 teaches that the same physical separation occurs when fluidized granular material flows in an enclosed, horizontal duct: expansion of the fluidized material from its settled state is small; the material flows in a “dense-phase”layer below a “supernatant” space within which fluidizing gas gradually accumulates; gas moves in this space at a speed significantly higher than the speed of the material layer. When employing a standard correlation for horizontal flow of air and water for estimating the pressure gradient required at a given point to sustain a desired flow of granular material, one should remember that not all gas reports to the supernatant space: some gas remains in interstices within the material.
It is evident that an improved system for horizontal conveying of a dry powder can be useful for tasks other than the conveying of heat. Lower pressure drop and a lesser requirement for conveying gas would be advantageous for the horizontal conveying of many economically significant powders, such as, for example, cement powder and grains like wheat, soy, rice, corn, etc., whose horizontal conveying is often a significant step either in charging the material to a process, committing it to long-term storage, or feeding it to means for its long-distance transport.
A fine powder ideal for use as a heat carrier is readily at hand, and, potentially, available at a low cost. Each year the petroleum industry discards spent FCC catalyst in the hundreds or thousands of tons. Fluidization by a gas places FCC catalyst in a liquid-like state displaying an effective “viscosity” smaller than that of water. When fluidized, the powder “imbibes” a moiety of gas, expanding just a bit in volume. The expansion greatly reduces the number of particle-particle contacts present within the powder at a given moment (accounting for its low effective viscosity). Derek Geldart supplied a classification of powders denoting their several distinct behaviors when fluidized by a gas (see Squires, Kwauk, and Avidan, Science, vol. 230, pp. 1329-1337, 1985). FCC catalyst is typical of the class of powders that Geldart designated “Group A”. In conventional parlance, a Group A powder would be called a “fine powder”; more appropriately, a Group A powder is called an “aeratable powder”.
Those skilled in vibration arts have long appreciated that vertical vibration at sufficient intensity confers a liquid-like character upon a granular medium (either “fine” or “coarse”). Thomas, Mason, Liu, and Squires (Powder Technology, vol. 57, pp. 267-280, 1989) reviewed engineering literature on vibrated powders from 1940 onward. The liquid-like character of a suitably vibrated granular medium resembles the character of this medium when fluidized by a gas. That is to say, when a stirring rod is introduced into either the vibrated or fluidized medium, application of a similar, small force is sufficient to move the rod from side to side; in both instances, the apparent viscosity of the medium is small, comparable to (or smaller than) the viscosity of water.
Moreover, like gas-fluidization, vertical vibration of FCC powder is capable of causing the powder to imbibe gas, the powder expanding just a little from its settled condition (see Thomas, Mason, and Squires, Powder Technology, vol. 111, pp. 34-49, 2000). That is to say, FCC powder is “vibrationally aeratable”.
Herein, a dimensionless “vibrational intensity” (K) denotes the ratio: maximum acceleration experienced by the powder from action of the vibration divided by the acceleration of gravity, g. For sinusoidal vibration, K=z0ω/g, where z0 is the maximum vertical displacement of the vibration (m); ω is angular frequency (s−1)=2πf; f=frequency (Hz).
Needs exist for chemically stable, easy-to-handle heat carriers and efficient heat carrying apparatus, suitable for use across a wide spectrum of temperatures.